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The more often young children are exposed to antibiotics, the greater their chances of becoming overweight. Researchers think it's because antibiotics disrupt the balance of helpful bacteria in the gut.

A study of nearly 65,000 American children shows that 69 percent are exposed to antibiotics before age 2. The more often they are exposed to the drugs, the more likely they are to be overweight or obese by age 5.

Researchers at the Children’s Hospital of Philadelphia and other institutions suspect that early exposure to antibiotics upsets the balance of helpful microbes that live in the children’s guts. Disrupting this personal “microbiome” is thought to slow a person’s metabolism, perhaps making them more likely to gain weight.

The researchers looked at electronic health records from 2001 through 2013 for children who had annual visits with a primary care doctor. They found that children were given antibiotics an average of 2.3 times each before age 5. Obesity risk increased with the number of antibiotic exposures, especially for children who had been given antibiotics four or more times.

Dr. Martin Blaser, the country’s foremost expert on the human microbiome, argues in his best-selling book “Missing Microbes” that the overuse of antibiotics, especially in young children, has led to the spread of metabolic and autoimmune diseases, including asthma, food allergies, eczema, obesity, and types 1 and 2 diabetes.

“Young children have critical periods for their growth, and our experiments are showing that the loss of friendly gut bacteria at this early stage of development is driving obesity, at least in mice,” Blaser wrote. “A key step in all of our approaches is to reduce the overuse of antibiotics in our children, starting now.”

The link between antibiotics and obesity in the new study held true only if the children were given broad-spectrum antibiotics, which are designed to wipe out all bacteria, not just the ones causing a particular illness. Doctors may choose broad-spectrum antibiotics over more targeted drugs if they don’t know what is causing an infection and want to treat the patient quickly.

“Narrow spectrum antibiotics, recommended as first-line treatment for common childhood infections, are not associated with obesity even after multiple exposures … This observation suggests a potentially modifiable risk factor for childhood obesity, given the relatively high use of broad-spectrum drugs,” the researchers wrote.

Antibiotic use was not the only risk factor for childhood obesity the researchers discovered during their study. Other risk factors include being male, living in an urban area, having publicly funded health insurance, being of Hispanic descent, being diagnosed with asthma or wheezing, and being given steroid medications.

Besides wiping out beneficial bacteria in the gut, the overuse of antibiotics in hospitals and doctors' offices is also leading to a rise in antibiotic-resistant bacteria like methicillin-resistant Staphylococcus aureus (MRSA).

Experts estimate that up to 50 percent of antibiotic prescriptions written in the United States every year are unnecessary. Every time a bacterium is exposed to an antibiotic, it has another chance to figure out how to outwit it.